I'm thinking of the blue stuff that you would use in your home....not the crack filler but the commercially applied stuff. It supposedly has an r value of 20-24 per inch and it's waterproof. You would have to shave it down level with the ribs if it was too thick.....
My first negative thought would be how do you remove it IF necessary in the future.

Hi Rob, if you do a search you should be able to find some threads on this. It seems that the general feeling is that along with the problem you mention, there is a lot of movement that would break the foam down over time, leaving you with some amount of dust in your walls.
Dave

I just got called over to a new friend's 60's Airstream for a look-see and someone had done the entire belly frame-floor area with sprayed on foam insulation.

I don't know if it was done in 1977 or 1997 - it had not disintegrated but with tupperware precision had held every bit of moisture it could against the wood and steel; shielded dry-rot safely away from any air circulation, wicked bathroom leaks quite offensively, and effectively sabotaged the thought of any painless access or repairs beneath the belly skins untill the day Joe-Somewhen does a shell-off floor replacement and complete sand blast of the frame.

On thinking long and hard on it I don't see a way to accommodate spray-in-foam with Airstream construction for all LONG TERM considerations and compromises....